DE 10 2006 006 871 A1 has disclosed a suspension strut for a motor vehicle having a vibration damper and having a supporting spring, and said document presents an auxiliary spring which adjoins the lower end of the supporting spring. The supporting spring is supported against an annular body, and the annular body is supported in a downward direction, toward the wheel carrier, on the auxiliary spring. If the vibration damper fully extends and, in the process, the supporting spring relaxes, the auxiliary spring may, if a residual preload in the supporting spring is undershot, relax such that the annular body against which the upper end of the auxiliary spring is supported can be pressed against the supporting spring. The auxiliary spring thus performs the task of slightly preloading the supporting spring even when the latter is in a fully relaxed state, such that, in particular, rattling noises owing to a supporting spring lifting off from the annular body and/or from the upper spring plate can be prevented. In particular, the auxiliary spring has the effect that the supporting spring, even in the fully relaxed state, reliably bears at one side against the annular body and at the other side against the upper spring plate. In this case, the annular body forms an intermediate disk spring between the supporting spring and the auxiliary spring.
The suspension strut furthermore has two piston-cylinder units which are arranged radially outside the vibration damper and radially within the supporting spring. By means of the units, the preload on the supporting spring can be varied, whereby the spring characteristic of the suspension strut changes, for example for a switchover of the driving characteristic of the vehicle from a comfort setting to a sport setting.
Furthermore, suspension struts having a height adjustment device are known, which are normally constructed from a cylinder element and a piston, wherein, in the cylinder element, there is formed a pressure chamber which can be charged with a pressure medium. In this way, the ride height of the vehicle body can be varied by virtue of the pressure chamber being charged with a pressurized fluid and by virtue of the piston being correspondingly moved in the cylinder element.
Suspension struts which have at least one auxiliary spring and in particular also a height adjustment device exhibit a considerable structural length, which often poses a problem with regard to the installation situation of such suspension struts. The disadvantage of the large structural length of such suspension struts arises in particular from the fact that the auxiliary spring and the supporting spring must be arranged together in series. Furthermore, there is the disadvantage that noise is generated if the auxiliary spring is arranged in an unprotected manner below the supporting spring and is exposed to environmental influences.